1. Field of the Invention
The present invention relates to a liquid crystal display including a ferroelectric liquid crystal material which is held between a pair of electrode substrates and whose optical response is asymmetric with respect to the polarity of a voltage applied from the electrode substrates, and a driving method for the display.
2. Description of the Related Art
A conventional liquid crystal display is of a holding type which continues to hold an image of a previous frame until a new image is written. The display has a problem that the phenomenon of blur occurs during display of a moving image, unlike an impulse type display such as a CRT which illuminates only for an afterglow time of a fluorescent material in each frame. In a case where one follows a moving object whose position changes between the images of successive frames, one observes the object as if it moves on the display while the image of the preceding frame is continuously displayed. The blur phenomenon is recognized as a result that the eyes tend to trace the moving object by finely sampling observable information so that the position of the object can be interpolated between the images of the preceding and succeeding frames.
In order to solve the problem, and obtain a sufficient display facility for the moving image in the liquid crystal display, it is preferable that high-speed response liquid crystals such as OCB (optically compensated bend) mode nematic liquid crystals and ferroelectric liquid crystals are used to provide an image display period and a blank display period in one frame. Concrete examples of such a preferable system have been proposed. In one known system, a back-light is momentarily lit each time a liquid crystal response is completed with respect to writing of the entire image for one frame. Moreover, a field alternation (field inversion) driving form (Jpn. Pat. Appln. KOKAI Publication No. 10076/2000) is also known, in which one frame is divided into first and second fields for the asymmetric polarity response property of the liquid crystal, a voltage of one polarity is applied in the first field to set the liquid crystal into a transmission state where transmission of light is controllable in an analog manner, and a voltage of the opposite polarity is applied in the second field to set the liquid crystal into a non-transmission state where light is hardly transmitted.
A monostable ferroelectric liquid crystal is known as the latter high-speed response liquid crystal having the asymmetric polarity response property. Mono-stability is obtained by polymer network introduced into a liquid crystal cell, or by an initial alignment treatment in which a slow-cooling process is carried out under application of a Direct Current voltage. Additionally, the asymmetric optical response can be obtainable even in a ferroelectric liquid crystal whose polarization property is symmetric, by means of polarization plates arranged properly. However, this liquid crystal is not suitable for the field alternation driving form since the DC voltage is applied to the liquid crystal cell on time average.
If the driving operation of writing and holding voltages via TFT devices or the like is repeated for each frame to drive pixels of the ferroelectric liquid crystal generally having the symmetric response property, a voltage drop may occur in each pixel during a holding period by dielectric relaxation since a response time of the liquid crystal is usually longer than a writing time. This pixel voltage drop lowers effectiveness of the written voltage, and this causes a problem that brightness and contrast ratio cannot be sufficient for the written voltage. Moreover, in a symmetric polarity alternation driving mode where the polarity of the voltage applied to the crystal is reversed for each frame so as to be positive or negative evenly, a “step response” phenomenon occurs after a certain frame in which the amplitude of the signal voltage is changed. In the phenomenon, the pixel is repeatedly switched between bright and dark states over several frames and finally set into a specified light transmittance (Verhulst et al.: IDRC'94 digest, 377 (1994)). This “step response” phenomenon is caused by a different factor from the blur phenomenon of the holding type display, but the moving object trailing an afterimage may be observed as if the blur phenomenon has occurred.
As a solution to the “step response” phenomenon, there is a technique of erasing or canceling the preset charge by performing a reset driving operation in which a constant voltage is applied before the writing of each frame. Conventionally, various methods and circuitries are proposed for the reset driving operation.
On the other hand, in a liquid crystal display having the asymmetric polarity response property, one frame is divided into two fields. For example, the display is driven in an alternating polarity driving mode where an image is written with a voltage of the positive polarity in the preceding field, and the image is erased with a voltage of the negative polarity in the succeeding field. In this case, the positive polarity is determined as a polarity in which the amount of change in the light transmittance is larger with respect to the voltage applied to the liquid crystal cell (i.e., the polarity in which the (ferroelectric) polarization of the liquid crystal cell is responsive or has a larger response). The negative polarity is determined as a polarity in which the amount of change in the light transmittance is smaller with respect to the voltage applied to the liquid crystal cell (i.e., the polarity in which the (ferroelectric) polarization of the liquid crystal cell is not responsive or has a smaller response). Additionally, when a DC voltage component remains in the liquid crystal cell, image sticking occurs due to uneven distribution of impurity ions caused by the DC voltage component. Therefore, it is general that the liquid crystal cell is driven with an AC voltage whose driving waveform has substantially the same amplitude in the positive and negative polarities so that no DC voltage component is applied. That is, the liquid crystal display having the asymmetric polarity response property can be driven by the voltage of substantially the same driving waveform except that a horizontal scanning frequency is double the frequency of the liquid crystal display having the symmetric polarity response.
However, in a case where the liquid crystal display with the asymmetric polarity response property is driven with the AC voltage whose driving waveform has substantially the same amplitude in the positive and negative polarities, the light transmittance increases in one or several frames after a certain frame in which the amplitude of the signal voltage is changed. When the amplitude is changed initially in the polarity of a larger response, the light transmittance increases at the time of rising. When the amplitude is changed initially in the polarity for a smaller response, the light transmittance increases at the time of falling. For example, when an available range of the light transmittance is divided into 64 brightness levels, deviation of at least one brightness level can be easily observed as the afterimage. This problem can be solved by the known reset driving operation for the liquid crystal display having the symmetric polarity response property. However, since one frame is divided into two fields, the writing time is regulated to half the normal writing time. Therefore, if a reset time is further disposed, writing deficiency is caused. Moreover, a time margin for resetting can be obtained by driving the scanning lines in units of two such that each scanning line pair is erased during the writing of other scanning lines. However, this method requires a complicated array structure and a reduced aperture ratio. If erasing is incomplete, non-uniform DC voltage components remain in the pixels. Although the asymmetric polarity response type liquid crystal display is easily operable as an impulse type display which displays a moving image at high speed, there remains the problem that the moving image is impaired due to an afterimage.